Silicoaluminophosphate (SAPO) molecular sieves contain a three-dimensional microporous crystal framework structure of [SiO2], [AlO2] and [PO2] corner sharing tetrahedral units. The [PO2] tetrahedral units are provided by a variety of compositions including phosphoric acid, organic phosphates such as triethyl phosphate, and aluminophosphates. The [AlO2] tetrahedral units are provided by a variety of compositions including aluminum alkoxides such as aluminum isopropoxide, aluminum phosphates, aluminum hydroxide, sodium aluminate, and pseudoboehmite. The [SiO2] tetrahedral units are provided by a variety of compositions including silica sols and silicon alkoxides such as tetraethylorthosilicate and fumed silica.
SAPO-34 and SAPO-18 have been reported as suitable catalysts for light olefin production from methanol. SAPO-34 belongs to the family of molecular sieves having the structure type of the zeolitic mineral chabazite (CHA). The preparation and characterisation of SAPO-34 has been reported in several publications, including U.S. Pat. No. 4,440,871; J. Chen et al. in “Studies in Surface Science and Catalysis”, vol. 84, pp. 1731-1738; U.S. Pat. No. 5,279,810; J. Chen et al. in “Journal of Physical Chemistry”, Vol. 98, pp. 10216-10224 (1994); J. Chen et al. in “Catalysis Letters”, vol. 28, pp. 241-248 (1994); A. M. Prakash et al. in “Journal of the Chemical Society, Faraday Transactions” vol. 90(15), pp. 2291-2296 (1994); Yan Xu et al. in “Journal of the Chemical Society, Faraday Transactions” vol. 86(2), pp. 425-429 (1990), all of which are herein fully incorporated by reference.
WO 98/15496 relates to a molecular sieve that is a mixed phase comprising silicoaluminophosphates of AEI and CHA structure, referred to as RUW-19. RUW-19 is said to produce an X-ray diffractogram that includes peaks at the following 2θ values: 9.3-9.5, 10.4-10.6, 12.7-12.9, 13.8-14.0, 15.9-16.1, 16.7-16.9, 18.9-19.0, 20.5-10.7, 21.0-21.3, 23.7-24.0, 25.7-26.0, and 30.9-31.1. This publication suggests that RUW-19 is different from a physical mixture of SAPO-18 and SAPO-34. RUW-19 has peaks characteristic of an AEI structure type molecular sieve, except that the broad feature centered at about 16.9 (2θ) in RUW-19 replaces the pair of reflections centered at about 17.0 (2θ) in SAPO-18 or AEI. Also, RUW-19 does not have the reflections associated with SAPO-34 or CHA centered at 17.8 (2θ) and 24.8 (2θ). According to WO 98/15496, RUW-19 is suitable for the conversion of methanol to olefins. Throughout this description, the XRD reflection values are referred to as (2θ), which is synonymous to the expression “degrees 2θ.”
U.S. Patent Publication Nos. 2002/0165090 and 2005/0096214 disclose a silicoaluminophosphate molecular sieve comprising at least one intergrown phase of molecular sieves having AEI and CHA framework types, wherein the intergrown phase has an AEI/CHA mass ratio of from about 5/95 to 40/60 as determined by DIFFaX analysis, using the powder X-ray diffraction pattern of a calcined sample of the silicoaluminophosphate molecular sieve. It also relates to methods for its preparation and to its use in the catalytic conversion of methanol to olefins.
U.S. Pat. No. 6,773,688 and U.S. Patent Publication No. 2004/0241072 disclose a process for manufacturing a SAPO-34 silicoaluminophosphate molecular sieve, the process comprising the steps of: (a) providing sources of aluminium, of phosphorus and of silicon, wherein the source of silicon is in solution with a water-miscible organic base; (b) forming a synthesis mixture from the sources; and (c) treating the synthesis mixture for a period of time and at a temperature sufficient to form the silicoaluminophosphate molecular sieve.
The present invention is related to a process for making a small particle size silicoaluminophosphate molecular sieve that comprises at least one intergrown phase of molecular sieves having AEI and CHA framework types, wherein the intergrown phase has an AEI/CHA mass ratio of from about 5/95 to 40/60 as determined by DIFFaX analysis, using the powder X-ray diffraction pattern of a calcined sample of the silicoaluminophosphate molecular sieve.